1. Field of the Invention
The present invention relates generally to a method and system for measuring the write current of a current mode write driver in a direct way, and also pertains to a current mode write driver, controlled by programming to produce a programmed write current, that precisely measures its actual write current produced in response thereto, such that the current mode write driver can be reprogrammed to calibrate its write current to a more accurate value. The current mode write driver is typically used in a tape or disk drive system to write data onto a recording tape or disk.
2. Discussion of the Prior Art
FIG. 1 shows a simple current mode write driver (as opposed to a voltage mode write driver) as is currently used in the art. For this current mode write driver, the current though the write head is determined by the value of the current source I1. For this current mode driver, the current through the write head L1 is not a function of the voltage V1, but is determined by the magnitude of the current produced by the current source I1. A typical tolerance for the current source I1 in an integrated write driver ASIC chip is 7 to 10%. This means that if the current source is programmed for a current value of 40 milliamps, then it could be as high as 44 milliamps or as low as 36 milliamps. If there were a way to precisely measure the write current directly, then a correlation table could be established to provide a more accurate current setting of the write driver.
In FIG. 1, the devices P3 and P4 are PMOS FET (PFET) devices, while the devices N1 and N2 are NMOS FET (NFET) devices, with FIG. 1 illustrating the connections of each device to its source s, drain d, gate g and also to the substrate Sb.
The current mode write driver has a first PFET P3 coupled to a first NFET N1 at a first node which is connected to a first side of a write head of the write driver, and a second PFET P4 coupled to a second NFET N2 at a second node which is connected to a second side of the write head, and defines an INODE at an I/O (input/output) pin where a source of the first NFET is coupled to a source of the second NFET and are coupled to a programmed current source.
For a normal writing operation, the inputs IN1P/IN1N are opposite to the inputs IN2P/IN2N. For example, if IN1P and IN1N are at a zero voltage potential, while IN2P and IN2N are at a potential V1, then the FET devices P3 and N2 are on and have a very low resistance, while the FET devices P4 and N1 are off and have a very high resistance. This causes the write current I1 to flow in the direction from V1 through P3, L1, and N2. Conversely, if IN1P/IN1N are at a voltage level of V1 while IN2P/IN2N are at a zero voltage potential, then the devices N1/P4 are on and the devices N2/P3 are off. This causes the write current I1 to flow in the direction from V1 through P4, L1, and N1.